The invention relates to a method for maintaining synchronization in a radio communications system. A synchronous random access operation may be used in mixed contention-based and contention-free modes for uplink time synchronization and/or power control.
In radio communications systems, a situation when a mobile station has temporary no data to transmit and/or receive, i.e. it is inactive, arise. However, it is desirable that the mobile station can quickly restart its transmission and/or reception. Such short reactivation time of less than 50 ms is defined as one of the fundamental requirements of currently standardized E-UTRA, see for example the technical specification 3GPP TS 25.913. It is assumed that user terminals (UE—User Equipment) will be kept synchronized with the network for a certain time even when not being in an active state anymore. More specifically, the UE, e.g. a dormant user terminal in a so called RRC Connected state, shall be kept uplink (UL) time synchronized in order to allow for a fast restart of transmissions and avoid time consuming non-synchronized random access procedures. During such inactivity time, the UE may be configured by the network with DRX/DTX (Discontinuous Reception/Transmission) cycles. Moreover, due to efficiency reasons it may be desirable that after some time of inactivity, which may depend on the used service, priority etc., the UE is allowed to lose the UL time synchronization, and in this case in order to obtain a timing advance or to request resources, the UE must use so called non-synchronized random access mechanisms.